Orthopedic cushions are used inter alia for cushioning between orthopedic aids and parts of the body, for example for cushioning between prosthetic parts, orthosis parts, or frame parts and the body of the user. Among the orthopedic cushions here are in particular those known as liners and sockets.
An orthopedic liner is a cover which has a cushioning or shaping function, for the distal end of the stump of a limb, or of a limb. The term liner is generally also understood to mean any material which has a cushioning and shaping function and takes the form of a stocking, of a cover, of a band, or of a patch, intended to cushion the boundary between a body part and an orthopedic means, e.g. a prosthetic socket, a support device, or an orthotic.
In particular the stump liners must fit well, like stump stockings, and for application they are therefore turned inside out and rolled onto the stump. This exposes the liner material to severe mechanical stress, and it has to be flexible and highly elastic. The liners are exposed to high tensile forces when they are pulled onto the stump. For the patient, the procedure can be very arduous and difficult. Application aids and application sprays are therefore available, but the latter can be hazardous to health.
In order to reduce friction levels on the external side of the liner, some known liners provide (friction-reducing) coatings which increase slip capability. However, the mode of action of a coating that provides slip is often based on smoothing at the surface, and the layer thicknesses in which the known coatings have to be applied are relatively high. The coating can then impair those properties of the liner material that are desired for its functioning. This also applies in principle to other types of cushion.
WO 03/051241 A1 discloses an elastomer liner coated with poly(para-xylylene). Said coating is applied by chemical vapor-phase deposition (CVD) and features excellent slip capability. It bonds well to silicone and to unfilled synthetic rubbers. The CVD technique can provide a relatively thin application which impairs the properties of the liner material to a smaller extent and provides economies in the use of the coating material.
The polyparaxylylene coating is a coating which increases slip capability, is solvent-resistant, and chemically inert, and is used in a very wide variety of applications, inter alia on electronic components and in medical technology. Polyparaxylylenes are polymers of a divalent xylene unit having the structure —CH2—(C6H5-n)Rn—CH2— where n=1 to 3, and with various possible substituents R, inter alia chlorine, fluorine, and amine. Said polymers are currently deposited in a standard CVD process from the pyrolyzed dimers on a very wide variety of surfaces, and inter alia on orthopedic cushions, for example liners.
There are also other known coatings intended for liners and having slip capability, made of polytetrafluoroethylene and of other dry-slip films.
It has now been found that although liners made of silicone can be coated with dry-slip films with the aid of CVD processes, the coating of other plastics cushion materials is however difficult, particularly when these are fluid-filled materials. Coating of filled elastomers, of elastomer gels, in particular polyurethane gels, or of plasticized thermoplastics (flexible plastics, for example flexible PVC), and also of hydrophilic polyurethane generally, is not possible with known CVD- or PVD-coating processes, or cannot be achieved with the desired quality.
In particular, it has not hitherto been possible to coat flexible polyurethanes and polyurethane gels with thin layers of dry-slip films. Polyurethane liners, in particular polyurethane gel liners and flexible polyurethane liners, are therefore frequently merely treated with sprays and with liquid or soft lubricants. A coating method using TPU (thermoplastic polyurethane) derived from solution does not give satisfactory quality. When polyurethane liners are coated with non-polyurethane-based, unpolar and, respectively, non-hydrophilic polymers, the results are non-uniform deposition, variations in layer thicknesses, heterogeneity, cracking, and large areas of non-adhesion. The liners thus coated are useless, and even if they could actually be used they exhibit premature wear, or a high level of wear.
The same also applies to coatings intended for application on other orthopedic cushions.